In fact, the NSA was monitoring all telephone calls. They were not intercepting all calls, since, for most calls the monitoring consisted only of recording the number called and the duration of the call. Thus Duffy (and many more prominent others) lied when a careful choice of words would have created the same false impression without a lie.

In particular "monitor" sounds less threatening than "intercept" or "eavesdrop" so they denied monitoring, which was taking place. This shows that they haven't noticed that the intelligence community is leaking like a sieve. It also shows an extreme short term bias. Finally it shows complete contempt for the true, but we knew that.